The present invention relates in general to a digital coding method and apparatus employing concatenated trellis coded modulation and linear block codes.
Trellis coded modulation (TCM), sometimes known as convolutional coding, is widely used in many digital communication applications, including deep-space, digital satellite transmission, high speed telephone modems and networks, broadcast networks and recording systems. Linear block codes are well known codes which allow single or multiple error correction.
It is known to employ multiple coding schemes simultaneously to provide desired levels of error rate performance, otherwise known as "coding gain". For example, U.S. Pat. No. 5,258,987 to Wei, issued Nov. 2, 1993, discloses a multilevel coding scheme which uses both TCM and a particular linear block code known as a Reed-Solomon (RS) code. In that scheme, a first portion of the input data stream is trellis encoded, and the resulting encoded stream is used to identify, for each of a succession of symbol intervals, a particular one of a predetermined number of subsets of symbols of a predetermined constellation. The remaining, non-trellis encoded input data is encoded using an RS code whose output is used to select for transmission, a particular symbol from the identified subset.
The coding scheme disclosed in the Wei patent allegedly provides a number of advantages. For example, the patent asserts that the bandwidth efficiency can be made to approach that of a system in which the non-trellis encoded bits are not coded at all; errors due to impulse noise of any magnitude can be corrected; the transmission of data at various different bit rates is easily accommodated; and the multilevel coded symbols selected for transmission can be interleaved in order to provide enhanced immunity to channel input pulse noise and/or correlated noise. However, because Wei's coding scheme only applies the RS code to the non-trellis encoded bits, it cannot significantly improve the code's performance with respect to additive white Gaussian noise.
Other multilevel coding schemes have been proposed which address the additive white Gaussian noise problem. However, these coding schemes tend to impose too high a penalty in coding efficiency and bit rate for the increased noise performance. For example, one known multilevel coding scheme encodes a data stream with an RS encoder, and then divides the stream into two substreams, one of which is trellis encoded as in the scheme disclosed in the Wei patent. These schemes tend to be inefficient, however, because the RS encoding is applied to all bits of the data stream equally, including those which are not to be trellis encoded. For the most part, the non-trellis encoded bits require relatively little encoding, or do not require additional encoding at all. As a result, the bit rate of this coding scheme is unnecessarily reduced.